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I am not quite sure what you are behind; But you can get the bandwith if you know the the IOPS and the IO size.(andwidth =IOPS*IO size)
The maximum you can get out of 2* 1Gbps line is around 2*110MB considering you have configured your NFS clients to make best use of both the interfaes. Remember that the bandwidth you get mostly depend on the io size and io type (sequential versus random) etc.